1. Field of the Invention
The present invention relates to a method and system for changing control parameters in accordance with a state of a process in process control.
2. Description of the Related Art
In an adaptive control apparatus for monitoring and controlling a state of a process, a difference between an externally input set value SV and a controlled variable PV output from a process such as a plant, i.e., a difference value e (SV-PV) is acquired. For the difference value e, PID (Proportional plus Integral plus Derivative) control parameters are calculated, thereby acquiring a manipulated variable MV. The manipulated variable MV is supplied to the process. The process operates in accordance with the supplied manipulated variable MV to output the controlled variable PV.
The PID control parameters used for acquiring the manipulated variable MV must be set in advance. Therefore, an adaptive control apparatus using an auto-tuning method or a self-tuning method for optimally calculating PID parameters has been developed.
However, in the adaptive control apparatus, a unit for calculating PID control parameters is independent of a unit for monitoring and controlling the process. More specifically, when an identification signal is supplied to the process, dynamic characteristics of the process are identified by analyzing a response signal output from the process, and the PID control parameters are calculated in accordance with the identification result. The set PID control parameters are not recalculated unless the process is changed.
Therefore, when a control system including the process is oscillated or vibrated or the difference value e is increased due to a sudden change in set value SV or a disturbance, the manipulated variable MV obtained by using the initially set PID control parameters is supplied to the process. As a result, a considerable time may be required until convergence of the control system, or the control system can no longer converge. In this manner, in order to maintain a normal control system, monitoring of the process and set of PID control parameters must be performed in correspondence with each other.
Meanwhile, when a controlled variable PV sampled for a predetermined cycle and supplied from the process exceeds a predetermined value, an adaptive control apparatus for recalculating PID control parameters has been developed. However, since calculations of PID control parameters require much time, it cannot immediately cope with a sudden change in state of a control system.
The conventional adaptive control apparatus does not perform control processing against a response error which takes much time until the controlled variable PV converges to the set value SV.
As can be seen from the above description, a demand has arisen for an adaptive control apparatus which can change PID control parameters in accordance with a change in state of a process.